English: 1.2 megavolt Cockcroft-Walton particle accelerator (background) at the Clarendon Laboratory, Oxford University, UK in 1948. It consists of a 6 stage voltage multiplier stack with a large globe to store charge at the top. The black segments of each column contain capacitors to store the charge, while the diagonal "crossrungs" contain high voltage vacuum tube rectifiers called kenotrons, which only conduct current in one direction. An alternating voltage of several hundred kilovolts is applied between the bottom columns. The columns function as a charge pump, pumping electric charge up the columns until the top terminal is at a potential of 1.2 million volts. The voltage is applied to opposite ends of an evacuated accelerator tube (not shown) which accelerates charged subatomic particles traveling through it to high speeds. All the metal parts of the machine must be covered by smooth curved metal "corona rings" to prevent corona discharge, which can cause charge to leak into the air. The Cockcroft-Walton accelerator was invented by British physicists John Douglas Cockcroft and Ernest Thomas Sinton Walton in 1932.